668         Mr.  Gr.  B.  Dyke  on  the  Use  of  the  Cymometer 
The  instrument  consists  of  an  ebonite  block  A  (fig.  1, 
p.  667),  from  the  sides  of  which  project  lugs  BB  which  are 
used  for  connexion  to  the  cymometer.  To  these  lugs  are 
soldered  brass  rods  CC,  each  about  3  inches  long,  and  between 
the  extremities  of  these  rods  is  stretched  a  fine  platinoid  wire  a 
(diameter  *05  mm.)  having  a  resistance  of  3' 5  ohms.  To 
the  middle  of  this  wire  the  thermojunction  is  soldered. 
This  junction  consists  of  two  very  fine  wires,  h,  e,  one  of 
pure  iron  (diameter  "20  mm.)  and  the  other  of  bismuth 
(diameter  '17  mm.)  ;  these  are  attached  to  the  platinoid  with 
special  solder  of  low  fusing- point,  the  contact  area  being 
made  as  small  as  possible.  The  other  ends  of  the  wires  are 
connected  to  the  galvanometer  by  a  flexible  cord.  The 
junction  is  shielded  by  the  ebonite  cap,  D,  which  screws  over 
the  plug  A.  The  electromotive  force  of  the  couple  is 
observed  by  means  of  a  low-resistance  Paul  single-pivot 
galvanometer,  having  a  resistance  of  4*88  ohms  and  a  figure 
of  merit  of  19 '5  microvolts  per  division. 
The  method  of  calibrating  the  junction  is  as  follows  : — 
For  wires  so  fine  as  the  platinoid  used,  the  high-frequency 
resistance  is  the  same  as  that  for  low-frequency  or  continuous 
current.  Hence  it  is  only  necessary  to  connect  up  the  hot- 
wire ammeter  in  series  with  an  adjustable  resistance  and  a 
secondary  cell,  and  to  pass  currents  of  known  strength  through 
it,  observing  the  corresponding  deflexions  of  the  galvanometer. 
These  observations  enable  a  curve  to  be  plotted  from  which 
the  root-mean- square  (or  equiheating)  value  of  the  current 
in  the  cvmometer  for  any  deflexion  can  be  readily  read  off. 
For  the  instrument  described  the  calibration  curve  is  such 
that  the  deflexion  varies  as  the  l*9th  power  of  the  current. 
An  auxiliary  resistance  is  also  required  and  is  constructed 
similarly  to  the  hot-wire  ammeter  just  described,  except 
that  the  thermojunction  is  omitted  and  the  resistance  of  the 
fine  platinoid  wire  is  7*2  ohms.  This  resistance  is  arranged 
so  that  when  required  it  can  be  put  into  a  second  gap  cut  in 
the  cymometer  bar,  a  short-circuiting  strip  being  used  to 
complete  the  circuit  when  it  is  not  in  use.  It  will  be  seen 
that  if  the  cymometer  bar  is  placed  in  the  proximity  of  a 
circuit  in  which  oscillations  are  taking  place,  the  value  of 
the  R.M.S.  current  induced  in  it  can  be  determined  by 
means  of  the  hot-wire  ammeter  described  above  for  any 
position  of  the  cymometer-handle  ;  that  is,  for  any  oscillation 
constant  or  any  frequency  of  the  oscillations  in  the  cymometer 
within  the  range  of  the  instrument. 
From  observations  of  this  R.M.S.  current  and  frequency, 
it  is  possible  to  deduce  the  logarithmic  decrements  of  both 
primary  and  secondary  circuits.    The  logarithmic  decrement  of 
